Discharge from an elevated ridge to a deep regional aquifer: a case study from Mt. Hermon, Middle East

Abstract

As is the case for many high-elevation karstic ranges in the world, the water balance of the active Jurassic aquifer in Mt. Hermon is highly speculative. For the latter, this is due mostly to large uncertainty in the infiltration coefficient. Consequently, until recently, when the new deep Shamir wells (up to 1,580 m depth) were drilled, water flow beyond the natural discharge remained unknown. The new wells provide evidence for leakage from Mt. Hermon southward, towards a huge reservoir in the deep Jurassic aquifer. This newly identified thick and confined aquifer extends under the entire Golan Heights and eastward. It is characterized by long residence time, depleted isotopic composition (δ18O and δD of −7.1 and −38.1‰, respectively), low Cl (~18 mg/L) and high SO4 concentrations (~650 mg/L), with elevated δ34SSO4 (~22‰). This composition suggests dissolution of marine gypsum of the Upper Triassic at the summit of Mt. Hermon. Another water source in Mt. Hermon is represented by the SO4-poor (~10 mg/L) Dan Spring, which discharges at the southwestern end of the mountain. Mixtures of these two end members at variable ratios determine the composition of all other Jurassic water sources around Mt. Hermon. The current study exemplifies how geological, hydrological and geochemical insights obtained from well-designed deep wells can dramatically improve the understanding of the flow regime from a high-elevation karstic ridge downstream. The study also has a practical aspect, whereby production from this deep system is mostly at the expense of the huge storage under the Golan Heights.